Facing Unexpected Reactivity Paths with ZrIV-Pyridylamido Polymerization Catalysts

A Computational Evaluation of the Steric and Electronic Contributions in Stereoselective Olefin Polymerization with Pyridylamido-Type Catalysts

A density functional theory (DFT) study combined with the steric maps of buried volume (%V_Bur) as molecular descriptors and an energy decomposition analysis through the ASM (activation strain model)-NEDA (natural energy decomposition analysis) approach were applied to investigate the origins of stereoselectivity for propene polymerization promoted by pyridylamido-type nonmetallocene systems. The relationships between the fine tuning of the ligand and the propene stereoregularity were rationalized (e.g., the metallacycle size, chemical nature of the bridge, and substituents at the ortho-position on the aniline moieties). The DFT calculations and %V_Bur steric maps reproduced the experimental trend: substituents on the bridge and on the ortho-positions of aniline fragments enhance the stereoselectivity. The ASM-NEDA analysis enabled the separation of the steric and electronic effects and revealed how subtle ligand modification may affect the stereoselectivity of the process.

Pyridylamido hafnium complexes with a silylene bridge: synthesis and olefin polymerization

Pyridylamido hafnium complexes with a silylene bridge were synthesized, characterized with X-ray crystal structure analysis and shown to polymerize 1-hexene and propene and copolymerize ethylene and 1-octene.

Ru-Catalysed synthesis of fused heterocycle-pyridinones and -pyrones

Heterocycle-pyridinones and heterocycle-pyranones have been prepared by Ru-catalysed oxidative coupling of N-unprotected primary heterocycle-amides and heterocycle-carboxylic acids with internal alkynes.

Transition metal complexes bearing tridentate ligands for precise olefin polymerization

This review covers key developments in the design of post-metallocene transition metal complexes (precatalysts) bearing tridentate chelating ligands and their application in olefin polymerization.

Imido-pyridine Ti(IV) compounds: synthesis of unusual imido-amido heterobimetallic derivatives

The reaction of lithiated picolines and [TiCl3(η(5)-C5Me5)] leads to several bridging or terminal imido compounds, each of which can be selectively formed by controlling the stoichiometry and temperature. Specifically, the dinuclear imido-bridged [TiCl(η(5)-C5Me5)(μ-NR)]2 (1a, NR = 2-imido-3-picoline; 1b, NR = 2-imido-5-picoline) species and the unusual Ti-Li imido-amido heterobimetallic complex [{Li(THF)}{Ti(η(5)-C5Me5)(NR)(NHR)2}] (2a, NR = 2-imido-3-picoline; 2b, NR = 2-imido-5-picoline) were isolated. Compounds 2 are in effect the first structurally characterized examples of titanium(IV) coordinated to terminal imido-pyridines. DFT-D calculations for 2a denote a multiple bond character between titanium and the imido ligand and a strong polarization of the electron density by the alkali cation in spite of the lack of intermetallic bonding.

Chelating σ-Aryl Post-Metallocenes: Probing Intramolecular [C-H···F-C] Interactions and Unusual Reaction Pathways

Our interest in chelating σ-aryl ancillary ligands was motivated by their potential to impart unusual reactivity, since we envisioned that σ-donors with minimal π-donation would create a catalytic center with enhanced electrophilicity. We developed a family of Group 4 post-metallocene catalysts supported by pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands bearing a fluorinated moiety in the vicinity of the metal. Notable features of these meta-substituted tris(hetero)aryl frameworks include their coordination geometry and inherent rigidity. For the first time, the elusive C-H···F-C interaction was manifested as NMR-discernible (1)H-(19)F coupling in solution and characterized by a neutron diffraction study. Their existence carries implications for catalyst design and in the context of weak attractive ligand-polymer interactions (WALPI), since they substantiate the practical viability of the ortho-F···H(β) ligand-polymer interactions proposed for living Group 4 fluorinated bis(phenoxyimine) catalysts. In metal-catalyzed olefin polymerization reactions, the notion of noncovalent interactions between an active ancillary ligand and the growing polymer chain is new. These interactions must be fragile and transient in nature, otherwise the intrinsic chain propagation process would be disrupted, and inherently tunable attractive forces such as hydrogen bonds are ideally suited to this role. The nature, relevance, and usability of extremely weak hydrogen bonds such as C-H···F-C has been a topical yet controversial area of research. We subsequently prepared a series of Group 4 complexes supported by fluorinated (σ-aryl)-2-phenolate-6-pyridyl [O,C,N] ligands. [(1)H,(19)F]-HMBC NMR experiments were conducted to probe the observed (1)H-(19)F coupling, and specifically separate contributions from scalar (J) coupling and cross-correlation (CR) interference. For the first time, a significant scalar component was confirmed for the (1)H-(19)F coupling in Ti-[O,C,N] and [O,N,C] complexes, which occurs with chemical connectivity across intramolecular C-H···F-C interactions. This result is important because the applicability of weak attractive ligand-polymer interactions in catalysis is feasible only if the observed coupling and hence the noncovalent interaction is genuine. The verified intramolecular C-H···F-C contacts in these complexes can therefore be considered as synthetic models for ligand-polymer interactions in olefin polymerization processes. Significantly, reports concerning late transition metal systems have appeared that hint at the generality of the WALPI concept for modulating polymerization reactions. We evaluated the olefin polymerization reactivity of Ti-[O,N,C] catalysts through judicious substitution. DFT calculations, which revealed diverse kinetically competitive reaction pathways and active sites (including unusual ethylene-assimilated species) in addition to normal chain propagation, were also employed to rationalize polymerization efficiencies. Further developments in catalytic applications of multidentate σ-aryl ligand systems and novel reactivity of the corresponding complexes can be envisaged.

NMR spectroscopy and X-ray characterisation of cationic N-heteroaryl-pyridylamido Zr(IV) complexes: a further level of complexity for the elusive active species of pyridylamido olefin polymerisation catalysts

New [(N(-),N,N(-))ZrR2] dialkyl complexes (N(-),N,N(-)=pyrrolyl-pyridyl-amido or indolyl-pyridyl-amido; R=Me or CH2Ph) have been synthesised and tested as pre-catalysts for ethene and propene polymerisation in combination with different activators, such as B(C6F5)3, [Ph3C][B(C6F5)4], [HNMe2Ph][B(C6F5)4] or solid AlMe3-depleted methylaluminoxane (DMAO). Polyethylene (M(w)>2 MDa and M(w)/M(n)= 1.3-1.6) has been produced if pre-catalysts were activated with 1000 equivalents of DMAO (based on Al) [activity >1000 kg(PE)(mol([Zr]) h mol atm)(-1)] or by using a higher pre-catalyst concentration and a mixture of [HNPhMe2][B(C6F5)4] (1 equiv) and AliBu2H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre-catalysts were treated with an excess of AliBu2H prior to addition of DMAO, which led to highly isotactic polypropylene ([mmmm]>95%). Neutral pre-catalysts and ion pairs derived from their activation have been characterised in solution by using advanced 1D and 2D NMR spectroscopy experiments. The detection and rationalisation of intercationic NOEs clearly showed the formation of dimeric species in which some pyrrolyl or indolyl π-electron density of one unit is engaged in stabilising the metal centre of the other unit, which relegates the counterions in the second coordination sphere. The solid-state structure of the dimeric indolyl-pyridyl-amidomethylzirconium derivative, determined by X-ray diffraction studies, points toward a weak Zr···η(3)-indolyl interaction. It can be hypothesised that the formation of dimeric cationic species hampers monomer coordination (especially of less reactive α-olefins) and that addition of AliBu2H is crucial to split the homodimers.

Cationic Group-IV pincer-type complexes for polymerization and hydroamination catalysis

Neutral Zr(IV) and Hf(IV) dimethyl complexes stabilized by unsymmetrical dianionic {N,C,N'} pincer ligands have been prepared from their corresponding bis-amido complexes upon treatment with AlMe3. Their structure consists of a central σ-bonded aryl donor group (C) capable of forming robust M-C bonds with the metal center, enforced by the synergic effect of both the coordination of peripheral donor groups (N) and the chelating rigid structure of the {N,C,N} ligand framework. Such a combination translates into systems having a unique balance between stability and reactivity. These Zr(IV) and Hf(IV) dimethyl complexes were converted in situ into cationic species [M(IV){N(-),C(-),N}Me][B(C6F5)4] which are active catalysts for the room temperature (r.t.) intramolecular hydroamination/cyclization of primary and secondary aminoalkenes as well as for the high temperature ethylene-1-octene copolymerizations.

Group IV organometallic compounds based on dianionic "pincer" ligands: synthesis, characterization, and catalytic activity in intramolecular hydroamination reactions

Neutral Zr(IV) and Hf(IV) diamido complexes stabilized by unsymmetrical dianionic N,C,N' pincer ligands have been prepared through the simplest and convenient direct metal-induced Caryl-H bond activation. Simple ligand modification has contributed to highlight the non-innocent role played by the donor atom set in the control of the cyclometallation kinetics. The as-prepared bis-amido catalysts were found to be good candidates for the intramolecular hydroamination/cyclization of primary aminoalkenes. The ability of these compounds to promote such a catalytic transformation efficiently (by providing, in some cases, fast and complete substrate conversion at room temperature) constitutes a remarkable step forward toward catalytic systems that can operate at relatively low catalyst loading and under milder reaction conditions. Kinetic studies and substrate-scope investigations, in conjunction with preliminary DFT calculations on the real systems, were used to elucidate the effects of the substrate substitution on the catalyst performance and to support the most reliable mechanistic path operative in the hydroamination reaction.